FIGS. 1A and 1B show an outline structure of a first conventional key switch 100. FIG. 1A is a plan view of the key switch 100, and FIG. 1B is a sectional view taken along the line IB—IB in FIG. 1A. FIG. 2 is a diagram showing a state where the key switch 100 is pressed. For example, the conventional key switch 100 used for a push button of a portable phone includes a base plate 6 provided with an annular wiring pattern 2, and with a wiring pattern 4 which is disposed on substantially a central portion of the wiring pattern 2 and which is separated from the wiring pattern 2. The key switch 100 also includes a key switch diaphragm 102 which is curved in a direction in which the central portion is separated away from the base plate 6 and which is disposed on the base plate 6, and a cover film 10 covering the key switch diaphragm 102 and the base plate 6. When the key switch 100 is used for the cellular phone and the like, the key switch 100 is generally required to have a durability to withstand million times presses.
One of surface (surface on the side of the base plate 6) of the cover film 10 that is in contact with the base plate 6 and the key switch diaphragm 102 has adherence. Thus, the key switch diaphragm 102 is not displaced in a direction along the surface of the base plate 6.
The edge of the key switch diaphragm 102 forms a contact 12. In a state where the key switch diaphragm 102 is placed on the base plate 6, the contact 12 and the wiring pattern 2 formed on the base plate 6 are electrically contacted with each other.
As shown in FIG. 2, a central portion of the key switch diaphragm 102 is pressed toward the base plate 6, the central portion of the key switch diaphragm 102 is dented toward the base plate 6, a contact 104 of the key switch diaphragm 102 formed in a recessed side of the central portion and the wiring pattern 4 formed on the base plate 6 are electrically connected to each other, and the wiring pattern 2 and the wiring pattern 4 are electrically connected to each other.
The key switch diaphragm 102 is formed by working or machining a metal thin plate having flexibility and conductivity. The key switch diaphragm 102 has a truncated pedestal 16 having appropriate height, and a spherical domical portion 18 being raised toward an upper portion of the pedestal 16 on the side of the upper portion of the pedestal 16 (constricted side of the pedestal).
When the key switch 100 is pressed, the pedestal 16 of the key switch diaphragm 102 is not deformed almost at all, and the domical portion 18 is mainly elastically deformed until the contact 104 and the wiring pattern 4 of the base plate 6 come into electric contact with each other. Thus, as compared with a case where there is no pedestal 16, greater pressing stroke and greater pressing force can be secured and with this structure, a good click feeling can be obtained.
When the key switch 100 is pressed toward the base plate 6 or this pressing is released as shown in FIG. 2, the volume of space surrounded by the base plate 6 and the key switch diaphragm 102 is changed. In order to change the air pressure in the space, a through hole 20 is formed in the cover film 10 in the vicinity of the edge of the key switch diaphragm 102. Since air passes through the through hole 20, the air pressure in the space when the key switch 100 is pressed or the pressing is released is maintained substantially constantly.